The use of a high boiling point volatile solvent for extracting coal is well known. The resulting extract contains some undissolved coal as well as shale particulates and other matter. These entrained solids have heretofore been removed from the liquid extract by passing the extract through a filter, and improved filtration has been achieved by the use of filter aids both as a precoat and as a body feed. Conventional filter aids, such as diatomaceous earth, have been successfully used for this purpose, but the cost of the filter aid is high and subsequent disposal of the used filter aid presents serious economic and ecological problems.
U.S. Pat. No. 4,301,015 describes a coal extraction process using a filter aid derived from the filter cake produced during the filtration of the coal extract. In accordance with the process described in this patent the filter cake is removed from the filtration apparatus and pelletized. The filter cake pellets are then heat treated in the presence of oxygen at temperatures in the range of 600.degree. C. to 950.degree. C., to calcine the pellets. The calcined pellets are then broken up into small particles and subsequently used as a filter aid in the coal extraction process. An optional process step described in this patent is to preheat the pellets to a temperature of about 400.degree. C. to drive off the coal extraction solvent prior to the high temperature calcination step.